The invention relates generally to communications systems and methods and, in particular, to a method and apparatus for canceling far end echo in a full duplex modem communications system.
A typical modem communications system has a plurality of stations, each of the stations having a modem. The modem has separate transmitting and receiving sections. Each modem is typically a two wire unit and, when the two wire connection reaches the telephone switching office, the signals are converted to a four wire system. The connection from the modem connected two-wire to the telephone switching office four-wire system is designated a near end connection while the connection from the switching system back to the two-wire path connecting to the remote modem is designated a far end connection. It is well known that there results, from the two-to-four and four-to-two wire conversions, a noise in the form of echoes which travel along the communication path and which distorts, and thereby causes errors to occur in, the signal reception process.
It is further well known that the echo resulting from the modem near end boundary as a result of a signal transmitted by the transmitter, the received near end echo, is, to a very good approximation, a linear function of the transmitted signal and is combined additively by the telephone line with the desired signal coming from the other modem to form a "composite" received signal. This echo is not affected by and does not exhibit frequency translation or phase jitter. Thus the received near end echo can be eliminated by linearly and adaptively filtering the transmitted signal and subtracting that adaptive filter output from the composite signal.
The echo returning from the far end boundary, as a result of a transmission by the transmitter of the modem, also distorts the signal received at the modem from a remote transmitter source. The received far end echo, however, is subject to both frequency and phase variations. And, while the far end echo received by a receiver is usually a small signal relative to the desired received signal, it nevertheless is often large enough that reliable reception is impossible if the far end echo signal is not canceled. When the far end echo is affected by frequency or phase variations, it is difficult to adjust a conventional echo canceler with sufficient speed and accuracy to effectively cancel the far end echo.
The conventional approach to canceling the far end echo contemplates subtracting an echo cancellation signal from the signal received from the two wire telephone line to produce a corrected, hopefully echo-free, receiver signal which is then processed by the receiver. The corrected signal is typically also employed by the feedback loop as an error update signal to adjust an adaptive linear filter which produces the cancellation signal from delayed samples of the transmit signal. The feedback loop error update signal, however, has a large "real receiver" signal and a relatively small echo signal. The relatively large "real receiver" signal to a large extent masks the desired error signal, that is, the remaining far end echo present on the line. As a result, adaptation of the echo canceler may not be sufficiently fast or accurate to track changes resulting from frequency translation and other phenomena.
To improve the echo cancellation signal generation process, various references use transversal filtering methods including signal rotation in connection with quadrature detection and equalization. The references also describe using the output of the decision threshold circuitry for controlling the transversal filtering process in generating the echo cancellation signal. Even so, however, the result of the echo cancellation circuitry has not been satisfactory.
An object of the invention is therefore an improved far end echo cancellation method and apparatus. Further objects of the invention are a method and apparatus for providing a representation of the transmitter output signal of a full duplex modem synchronized to the receiver's timing reference for use in canceling the far end echo signal mixed with the true receiver input signal. Other objects of the invention are a method and apparatus for effecting synchronization of the transmitter output samples to the receiver samples (even when neither modem is in loop-back timing), and for providing such synchronization without using an analog interpolation filter, and for employing threshold decision outputs from a receiver decision circuitry for controlling the phase and amplitude of a far end echo cancellation signal.